In a vehicle with an automatic transmission, a crankshaft is typically connected to the transmission using a torque converter. The torque converter includes a pump and a turbine. The pump is connected to the crankshaft and rotates at the same speed as the crankshaft. The turbine is connected to the transmission and rotates at the same speed as the transmission. The torque converter allows the crankshaft to spin somewhat independently from the transmission. If the engine is rotating slowly, the amount of torque transferred by the torque converter is low, which allows the vehicle to come to a stop while the engine continues to rotate. At steady vehicle speeds, the transmission and crankshaft operate at nearly the same speed, which transfers torque from the crankshaft to the transmission more efficiently. Some torque converters have control systems that regulate the amount of slip between the turbine and the pump when the vehicle reaches a desired speed to reduce slip and to increase fuel efficiency.
When the engine rotates, the crankshaft rotational speed may vary periodically as shown by a solid line 6 in FIG. 1. Crankshaft speed variation may be particularly high when the engine has a small number of cylinders. In addition, the rotational speed of the turbine is also likely to vary sporadically as shown by a dotted 8 line in FIG. 1. Variations in rotational speed may cause the crankshaft and turbine to periodically rotate at the same speed causing a converter clutch to periodically lock and unlock the pump and turbine together. Periodically locking and unlocking the converter clutch may damage to the clutch. Periodically locking and unlocking may also cause unacceptable levels of noise and vibration.